The present invention relates generally to an overvoltage protection device comprising a metal oxide varistor (MOV) that can modify its operating characteristics to protect a ground fault circuit interrupter during the occurrence of an overload voltage surge.
There is a need to protect AC line powered electronics from voltage spikes. One of the most popular methods of surge suppression is the metal oxide varistor (MOV). The surge suppression requirements for AC line powered electronics have increased in recent years. For example, UL testing now requires ground-fault circuit interrupters GFCIs to survive and function normally after several 6 kV, 3 kA voltage surges. It is also required that any surge suppression device for a GFCI must either survive, or self destruct in a safe manner, when a 10,000 amp current surge is applied. A MOV that is 14 or 20 mm in diameter is typically required to have the joule capacity to survive these high voltage, high current surges. At the same time however, circuit interrupter receptacles and plug-in units are required to be small for aesthetic and practical reasons. In particular, receptacle interrupters must fit into standard size wall boxes.
MOVs are bi-polar ceramic semiconductor devices. They act as an open circuit as long as the voltage across them is less than their maximum continuous operating voltage (MCOV). Above their MCOV, MOVs operate as non-linear resistors with a resistance that decreases as the voltage across them increases. This makes MOVs an effective solution for protecting downstream electronics from over-voltages and voltage transients. Since a MOV is open-circuit during normal operating voltages, it does not consume current. However, during voltages transients above the MCOV, it quickly shunts current away from the downstream electronics. In addition, since it is a bi-polar device it can protect electronics from negative and positive voltage transients.
A typical MOV consists of a ceramic mass of zinc oxide grains (mixed with small amounts of metal oxides) placed between two metal electrodes. The shape of the ceramic mass is typically a disk shape with the metal electrodes on the two flat surfaces, though larger MOVs can be square with rounded corners or even toroids. For smaller MOVs (53 mm diameter and less) a lead is soldered to each electrode and the MOV is encapsulated in epoxy or other insulative material.
Clearly there is a need for improved surge suppression techniques that take up a relatively small volume, clamp at low enough voltage to protect 120/240 v circuits adequately and are resistant to catastrophic failure during high current surges.
This is because a high voltage transient surge can totally or partially damage electrical devices such as Ground Fault Circuit Interrupters (GFCIs) located in homes, factories and commercial buildings. In many instances, the damage can cause only the protective features of the GFCIs to become either partially or fully inoperative while the device itself continues to conduct electricity. Thus, a user of this type of GFCI could still obtain power from the face terminals of a GFCI but not receive the GFCI type protection.
In operation, an MOV is connected in parallel with the device that is to be protected such as but not limited to a GFCI. At low voltages the MOV has a very high resistance. At high voltages, the varistor has a very low resistance so that when a high voltage transient surge appears on the power supply line, the MOV, which appears as a low resistance, prevents the transient voltage surge from reaching the device. As stated above, conduction through an MOV begins when the voltage across the MOV reaches a maximum continuous operating voltage, referred to as the varistor voltage. As the voltage increases, the MOV's resistance drops rapidly and may approach zero. Because the resistance of the MOV decreases as the voltage increases, the MOV diverts transient current through itself and not through the device that is connected in parallel with and down stream of the MOV. After the occurrence of the voltage transient surge, the MOV returns to its normal high resistance state and is ready for the next high voltage surge.
However, another characteristic of an MOV is that during operation, the MOV will increase in temperature as it conducts high voltage surges. If the voltage surges are well spaced, the MOV can cool down between events. However, if the events are closely spaced, the MOV will not have enough time to cool down and this heating of the MOV will allow additional current to flow through the MOV. The additional current will further raise the temperature of the MOV, and this will continue until the MOV destroys itself. This condition is known as thermal runaway. When in its thermal runway state, an MOV can explode and possibly cause extensive damage to surrounding components.
One way of protecting the MOV itself is with a thermal protection device wired in series with and located to be heated by the MOV element. The melting point of the thermal protection device is set to be at a temperature below that which will cause the MOV to enter its thermal runaway state. As the temperature of the MOV rises, a point will be reached where the thermal protection device will melt and disconnect the MOV from the circuit. When the circuit is a GFCI, it will no longer be protected by the MOV and the full impact of the high voltage transient pulse will be applied to the GFCI. Thus, when an overload condition occurs, the over voltage transient surge is free to be absorbed by the GFCI that was being protected.
The peak surge current rating of an MOV is a function of the area of the disc itself. To protect a GFCI from destructive high voltage transient surges, test have shown that a relatively large MOV is needed. Unfortunately, it is difficult to connect an MOV of this size to a GFCI and still fit the GFCI and the MOV into a single outlet box.
What is needed is an overvoltage device which can protect a circuit during an overload voltage surge, that is small enough to fit into a relatively small enclosure such as a single gang electrical enclosure.